upgrading – Do we upgrade core or PHP first?


Drupal 7 can run on any PHP version between PHP 5.6 and PHP 7.3. For running on PHP 7.4, it’s required at least Drupal 7.71.

Drupal 8 can run on any PHP version that is newer than PHP 5.6. For running on PHP 7.4, it’s required at least Drupal 8.8.3.

If the site is running a version of Drupal that is older than Drupal 7.71, I would first upgrade PHP to PHP 7.3, and then upgrade to Drupal 8.
If the site is running on Drupal 7.71 or any newer version, you can upgrade to PHP 7.4, if you mean to upgrade Drupal to Drupal 8.8.3 or newer versions.

I would also test the upgrade on a local test site, as the modules you are using could cause problems with newer PHP versions.
It should be safer to upgrade to PHP 7.3, as there are incompatible changes in PHP 7.4 that required changes in Drupal 7; it’s probable the same changes are required in third-party modules, but those changes are still to be applies to those modules.